Due to the vitamin D fortification program in the United States, it has been assumed that abnormal vitamin D nutrition is not a problem in old people. However, from the analysis of plasma 25-OHD concentrations in a group of 268 healthy free-living elderly people in the United States, we observed a 13.5 percent incidence rate for biochemically evident subclinical vitamin D deficiency. This research plan is designed to determine why the population of old people has a decreased plasma 25-hydroxyvitamin D concentration. Studies have been formulated to reveal the basis of the lowered 25-hydroxy-vitamin D levels and determine if the vitamin abnormality co-developed with changes in other calcium-homeostatic parameters, or occurred as an independent process. Genesis of the marginal vitamin D deficiency will be delineated by determining: 1) vitamin D nutritional status (food records with analysis by computerized nutrient-base program); 2) vitamin D biochemical status (measurement of plasma concentration for vitamin D and its hydroxylated metabolites); 3) efficiency of intestinal vitamin D and calcium absorption (double isotope methodology with calculation of rate constants); 4) concentration of the vitamin D binding protein (quantitated by rocket immunoelectrophoresis); and 5) mineral-metabolism activity (radioimmunoassay for plasma parathyroid hormone and nephrogenic cyclic-AMP). It is suggested from our preliminary results that low-vitamin D nutritional status is currently an unrecognized health concern for a substantial number of old people in the United States. If this observation is substantiated, then information from this investigation will serve as a touchstone for intervention studies into the prevention and treatment of age-related calcium-homeostatic diseases. And most importantly, it does not appear that a controlled study has been conducted in the United States on the influence of aging on vitamin D nutritional status.